The present disclosure relates to a cathode active material and a non-aqueous electrolyte secondary battery and, more particularly, to a cathode active material having two kinds of lithium cobalt composite oxides of different compositions and a non-aqueous electrolyte secondary battery using such a cathode active material.
In recent years, owing to the development of electronic apparatuses and the realization of their miniaturization according to the advancement of a semiconductor integrating technique, a demand for a battery serving as a power source of a portable electronic apparatus is rising. Characteristics which are requested to the battery are those of a secondary battery which is small and light weight, can be used for a long time, and can be charged and discharged.
As small secondary batteries having those characteristics, a nickel hydrogen battery, a nickel cadmium battery, a lithium ion secondary battery, and the like can be mentioned. Among them, the lithium ion secondary battery having the high voltage of the 4V grade and a high energy density has large electric power consumption. A demand for the portable electronic apparatus or the like to which the primary battery is rarely used is also increasing.
It is a feature of the lithium ion secondary battery that a battery having a large capacitance, that is, the large energy density can be formed by combining a cathode whose oxidation-reduction potential is higher and an anode whose oxidation-reduction potential is lower as compared with those of other batteries. At present, the lithium cobalt composite oxide is a main stream as a cathode active material of the lithium ion secondary battery.
Although the lithium ion secondary battery using the lithium cobalt composite oxide has such an advantage that an average discharge potential is high, cycle characteristics are not good and a voltage drop at the time of a large-output discharge at a low temperature is large. Therefore, there is such a problem that, for example, when a power source of a personal computer (hereinbelow, abbreviated to a PC) is turned on in a cold district, the PC is not activated due to an insufficient output.
For example, in JP-A-2001-319652, there has been disclosed a lithium cobalt composite oxide expressed by the following general formula.LiCoxAyBzO2 
(in the formula, A denotes at least one kind selected from Al, Cr, V, Mn, and Fe; B denotes at least one kind selected from Mg and Ca; x is set to a value within a range of 0.9≦x<1; y is set to a value within a range of 0.001≦y≦0.05; and z is set to a value within a range of 0.001≦z≦0.05)
According to the lithium ion secondary battery using such a cathode active material, although high-temperature characteristics can be improved, it is difficult to obtain the satisfactory characteristics with respect to load characteristics and low-temperature characteristics.
Further, for example, to improve the low-temperature output characteristics of the lithium ion secondary battery, a technique for adding zirconium Zr upon synthesis of the lithium cobalt composite oxide has been proposed. Moreover, to improve electrode filling performance, by mixing the lithium cobalt composite oxide added with zirconium Zr as coarse powder or micro powder and using it, the high capacitance of the lithium ion secondary battery can be realized.
However, if the lithium cobalt composite oxide added with zirconium Zr is mixed as coarse powder or micro powder and used, although the high capacitance of a cell can be realized, there is such a problem that the safety deteriorates remarkably. It is also requested to further improve the cycle characteristics and the low-temperature heavy load characteristics.
It is, therefore, desirable to provide a cathode active material which has a high capacitance and can improve cycle characteristics, low-temperature heavy load characteristics, and safety and to provide a non-aqueous electrolyte secondary battery using such a cathode active material.